Effects of prostaglandin F 2 alpha (PGF 2 alpha) and ICI 80,996 on secretion rates of progesterone and estradiol will be determined in intact cattle. Electromagnetic flow probes will be placed on both ovarian arteries and both ovarian veins will be cannulated. Ovarian and jugular blood samples will be collected at frequent intervals for a 24-hour period following treatment. The thermal conductivity of luteal tissue and ovarian stroma will also be monitored as a qualitative indicator of changes in regional ovarian blood flow. Radioactive microspheres will be introduced into the left ventricle at timed intervals to quantitatively estimate changes in regional ovarian blood flow. Accute effects of PGF 2 alpha and ICI 80,996 on the percent of cytoplasmic area occupied by electron dense secretion granules and lipid droplets will be determined. The role of estrogens in lyteolysis associated with parturition is to be investigated. Preliminary data suggest that corpora lutea at 250 days of gestation are responsive to LH while those at 270 days are not. The number of estrone implants will be increased between 230 and 250 days of gestation to mimic the increase in estrogen concentrations observed between 250 and 270 days. Corpora lutea will be removed 20 days after initiation of treatment and incubated with LH and cAMP.